"fallback", or alternately "failover"
E.g. "it may cause some clients to fallback to the next RADIUS accounting server."
Assuming the context is that the clients that would normally use this RADIUS accounting server end up using their next (less preferable) choice, that's usually called "fallback". "failover" also gets used, but more often in the context when all the clients would use the other server, not just some.
